Dual fuel: double trouble?

Mohamed Faquiri reveals how converting power stations to dual fuel is a challenge – but a rewarding one

The global power generation industry is under increasing strain to keep up with demand while also having to comply with more stringent environmental legislation.

A growing world population requires more power, and although fuel prices continue to be volatile, natural gas has recently become a more attractive option. This trend has led to a growing number of power stations looking at the possibility of modifying the power generating equipment to accept different fuel sources.

Many established power generation systems have used coal-fired stations to generate the base load of energy that is required, while oil and gas-fired units are held in reserve to meet peak demand for a few hours each day.

However, these back-up facilities are increasingly being used to meet daily demand as it continues to increase; this being the best solution until additional generating capacity can be commissioned.

Power stations that are fuelled by oil or diesel face considerable running costs, and most of these facilities are run at a loss in favour of keeping the lights on and preventing blackouts at times of peak demand.

However, extending this practice to cope with an increased base load is unsustainable, purely from a financial position, and now there is also added pressure to reduce emissions, which is increasing the urgency to find an alternative.

One of the prime considerations is the time required to resolve this escalating situation; a new conventional power station will have a lead time of two or three years and a nuclear installation could be at least nine or 10 years away.

A more immediate solution can be found in converting the liquid-fired turbine to a gas-fired version, a process that could be completed with the generator offline for less than two months.

Apart from the reduced fuel cost, the gas-fired turbine offers a number of additional advantages that will help to reduce the overall operating cost of the installation.

As a fuel, gas burns cleaner than oil, which means that the hot gas path (HGP) components should have increased intervals between inspections, when compared to the liquid-fired maintenance regime. Generally, the components within a gas turbine are exposed to less deposition and contamination, which can also extend the overall life of the components.

By increasing the periods between maintenance inspections, the availability of the generator is increased making it more productive and less costly, both in terms of direct maintenance costs and indirect loss of production.

Furthermore, the cleaner burn provided by the gas option produces less particulate and gaseous emissions, which can considerably reduce the environmental burden of the power plant.

The actual process of physically changing from oil to gas is relatively straightforward; it is the technology and engineering expertise that holds the key to a successful project. In essence, the conversion requires a number of changes to the HGP components including the mixing chambers and fuel nozzles as well as new coatings for the turbine blades and vanes.

This change to dual-fuel combustion allows a turbine to continue running on oil until such a point that the change to gas is required. In this way a complete set of turbines can be individually converted, minimising any disruption to power generation until such a point that the plant is ready to convert to gas. With the mechanical modifications completed, only the electronics and control software will remain to be changed before the transformation will be complete.

Of course, the OEMs can provide a solution, either in the form of an entirely new turbine, or as a conversion. However, there is a second option; specialist independent service providers that have decades of experience in maintaining and repairing rotating equipment, such as Sulzer, can often provide an attractive alternative.

By the nature of their work, OEMs concentrate on their core business, manufacturing new equipment. And although the replacement parts may be available, the process of getting a team on the ground may be rather drawn out.

The increased flexibility offered by a more local field service team can often present a more cost-effective solution.

Sulzer provides a full-service concept that includes the maintenance and repair of gas turbines, including the manufacture of new components, specialised machining operations and high-speed balancing.

Projects are supported by design engineering teams and experienced project managers, which enables the field service engineers to deliver a project on time and within budget with the assistance of local service centres that maintain a constant dialogue with the client.

One of the most important aspects of a project such as an oil-to-gas conversion is the planning. This forms the framework on which the rest of the project will be based and ensures that every aspect of a particular task is in place on time. Sulzer maintains this principle through its local service centres that act as the single point of contact for the client. In this way any challenges that arise during a project can be quickly resolved without impeding the progress of the project.

As both the economic and political pressures build, countries in the Middle East, Africa and Latin America, which have a high percentage of oil-fired power stations will soon be affected by the increasing pressure to reduce emissions. As the need for improved environmental sustainability increases, the decision to alter the fuel source for these power stations will be driven more by politics than economics.